The development of thin film transistors (TFTs) using printing techniques has uncovered a number of issues that should be addressed before large scale production of such devices becomes more prevalent. Typically, such devices have as one of its components a gate dielectric. So, for example, it has been found that a hydrophobic gate dielectric results in improved molecular ordering of the organic semiconductor and, therefore, improved performance in organic TFTs. To achieve such hydrophobicity (or low-surface energy), often a silane surface modification is implemented.
In this regard, Salleo et al. observed a significantly increased TFT mobility with a solution processed polyfluorene (F8T2) semiconductor on octadecyltrichlorosilane (OTS) treated silicon dioxide (Salleo, et al. App. Phys. Lett, 81, 2002, 4383). The same low-surface energy silane treatment was also effective with other organic semiconductors such as the polythiophene PQT-12. When treating a silicon dioxide surface with a silane, the silane reacts with hydroxyl groups on the silicon dioxide surface and a strong bond forms. The silicon dioxide surface often ends up functionalized with a monolayer of the silane. Hexamethyldisilazane (HMDS) is another substance that is often employed to render a silicon-based surface hydrophobic and improved transistor performance has been achieved with HMDS treated Si02 gate dielectric as well. The silane layers are often applied after the source and drain electrodes for the TFT have been patterned on the gate dielectric. Since they are only monolayer thin, this layer does not cause much contact resistance in a TFT.
However, this silane surface modification does not work well for many gate dielectrics that do not have abundant hydroxyl groups at its surface, such as many polymer dielectrics. A hydrophobic (polymer) layer can be deposited instead, but simply adding a hydrophobic layer also partially covers the source and drain electrodes and causes contact problems since such layer typically may be thicker than a monolayer.
Other gate dielectrics may be used, but many do not have functional groups which allow binding of the surface modifiers, such as OTS or HMDS. For example, on organic dielectrics such as Polyvinylphenol (PVP), epoxy resins, Polyvinylalcohol (PVA), etc., silane coatings are less effective.
As a replacement for silane surface modification, hydrophobic polysilsesquioxane (PSSQ) layers have been employed. Liu et al. (JACS communications, 128 (14) 4554-4555, 2006) demonstrated excellent TFT mobility using a PP-PMMA gate dielectric which was coated with a Poly(methyl silsesquioxane). The semiconductor was PQT-12 and the source-drain electrodes were gold nanoparticles deposited onto the dielectric by mask-assisted microcontact printing. The PSSQ forms a thin layer on the surface of the underlying material and the layer does not depend on covalent bond formation as in the case of silane surface-modifiers. The thickness of the PSSQ layer may be several nanometer (nm) up to 10 s of nm, or higher. This known use of PSSQ layers, however, does not address the problems with attaining suitable source and drain contacts in a thin film transistor fabricated by inkjet-printing, for example.